1. Field of the Invention
The present invention generally relates to semiconductor technology and device designs, and more particularly to an organic-based thin film transistor device and a method for producing the same.
2. Description of the Related Art
Thin film transistors (TFTs) with active organic layers and polymer-based electronic components are emerging as an inexpensive alternative to silicon-based TFTs for some applications. The use of organic and polymeric materials provides two main advantages. First, organic-based devices can be produced using a simpler and cheaper fabrication process, in contrast to the expensive equipment and processing associated with silicon processing. Second, it is possible to fabricate the devices on flexible plastic substrates, due to the ability to process organic materials at lower temperatures, and to the greater mechanical flexibility of organic-based components relative to inorganic materials such as silicon and conductive metals. However, despite considerable research and development effort, organic-based TFTs have not yet reached commercialization, at least in part due to relatively poor device characteristics of prior art organic TFTs.
Fabrication of an all-organic TFT requires various organic or organic/inorganic hybrid materials: semiconductors, insulators, and conductors. The conductor may be selected from conducting polymers such as polyaniline and poly(ethylene dioxide thiophene), and metal or graphite colloid particle-based inks. There are a variety of polymeric organic insulators that may be used, such as polyamide or PMMA for the semiconductor. Organic p-type (hole transporting) and n-type (electron transporting) materials are both known in the art and have been tested as the semiconductive channel in TFTs.
In general, circuitry using organic transistors have the potential of reduced power consumption and simplicity in the design. However, complementary circuitry using both organic N and P channels transistors are not common, for example, U.S. Pat. No. 5,625,199, the complete disclosure of which is herein incorporated by reference, teaches a technique to fabricate complementary circuits with inorganic n-channel and organic p-channel thin film transistors. Additionally, U.S. Pat. No. 5,936,259, the complete disclosure of which is herein incorporated by reference, describes a switch based on a thin film transistor design (TFT) using a fused ring organic compound as semiconductor. Furthermore, U.S. Pat. No. 5,804,836, the complete disclosure of which is herein incorporated by reference, describes an image processor design which operates on an array of polymer grid triodes. Similarly, prior art disclosures also teach a 5-stage ring oscillator using copper hexadecafluorophthalocyanide as the n-channel material and oligothiophenel oligothiophene derivative as the p-channel material.
Two popular structures of an existing polymer thin-film transistor are shown in FIGS. 9(a) and 9(b). These structures have two major disadvantages. First, there is a corner thinning problem due to topography, and second, the most sensitive portion of the body element is exposed to process induced contamination. The resulting devices have poor performance and inconsistent properties. Shown in FIG. 9(a) is the first typical structure of the polymer transistor. The source 11 and drain 12 are first patterned. Then the body material 13 is deposited and patterned. The body 13 is a semiconductive polymer or oligomer, and it is applied to the surface of the source 11 and drain 12 islands by evaporation, spin-coating, dip-coating or printing, depending on the organic semiconductor used.
The body material 13 is patterned in one of three ways. The most common method is by evaporation of the semiconductive material through a shadow mask. The other two methods are printing (i.e., screen printing or inkjet printing) and using conventional lithography by first applying a protective coating over the semiconductor, then applying the photoresist, patterning, and etching. A brief thermal anneal may be needed, depending on which type of organic semiconductor material is used. The last step is applying a protective coating to the semiconductor to passivate the devices from contamination.
After patterning the body portion 13, the substrate is wet cleaned. The body surface, especially in the channel region, deteriorates due to the unwanted chemical reaction. After a thermal treatment, the body element 13 becomes thin around the corners 16, 17 of the source 11 and drain 12 due to reflow. Typically, semiconductors decompose before melting. The source/drain 11, 12 to body contact area are significantly reduced as the result of the corner thinning 16, 17 of the body element 13. Then, the gate material 14 is deposited after a thin insulating polymer 15 is coated on top of the body element 13 and the exposed source 11 and drain 12 regions.
Another common structure of the polymer TFT structure is shown in FIG. 9(b). The gate 14 is formed first, then an insulating polymer 15 is coated thereon. Again, the corner thinning problem presented at the corners 16, 17 of the gate 14 causes the possibility of shorting of source 11 and drain 12 to the gate 14. After the source 11 and drain 12 are formed, the body element 13 is formed. In this case, since the body to channel interface is not exposed to any chemical, the resulting transistor yield and performance is better than those of the first transistor.
In both of the bottom-contact devices shown in FIGS. 9(a) and 9(b), there is a well-documented problem with ensuring good contact between the electrodes and the organic semiconductor. One approach used to solve this problem has been to modify the surface properties of gold electrodes using thin self-assembled monolayers, which improves wetting of the electrode by the organic semiconductor and may also decrease the chance of delaminating. However, the topography of the bottom electrodes may still hamper film formation and reduce the contact area. Therefore, there is a need for a new and improved structure and method for forming a polymer thin film transistor, which does not have the problems inherent with the prior art devices.